feat: Add MessageStore base class abstraction for message threads

[Ricardo-M-L]

Summary

Adds a storage abstraction for message_thread in teams, as proposed in #6227.

• MessageStore abstract base class with standard async CRUD operations (add_messages, get_messages, clear) plus save_state/load_state for persistence
• InMemoryMessageStore concrete implementation — the default, fully backwards-compatible backend with optional TTL (time-to-live) based message expiration
• Integration into BaseGroupChatManager and all team implementations (RoundRobin, Selector, Swarm, MagenticOne, GraphFlow) via an optional message_store constructor parameter
• Backwards compatibility — _message_thread remains accessible as a property that delegates to the underlying store; existing subclass code continues to work without changes
• 20 comprehensive test cases covering ABC contract, CRUD, TTL expiration, state persistence, and custom store implementations

Key design decisions

1. TTL as a constructor parameter — MessageStore(ttl=300) enables 5-minute message expiration; None (default) means no expiry
2. Zero breaking changes — when no message_store is provided, an InMemoryMessageStore is created automatically, preserving identical behavior to the previous List-based storage
3. Extensibility — users can implement custom MessageStore subclasses (e.g., Redis, database-backed) by implementing 5 abstract methods

Fixes #6227

Test plan

• Run pytest python/packages/autogen-agentchat/tests/test_message_store.py — 20 test cases for MessageStore abstraction
• Run existing team tests to verify backwards compatibility
• Verify InMemoryMessageStore with TTL correctly prunes expired messages
• Verify state save/load round-trip works with the new store

🤖 Generated with Claude Code

[Ricardo-M-L]

Closing — this feature should have been discussed in an issue first before submitting a large PR. Apologies for the noise.